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Help me solve a Bitcope problem with strange noise

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abouras

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Hi all!!!

I'm in this situation:
Installed 50MHz OSC module in U19
Insert ADC5540 module in 24 pin socket
Make sure 3 pin header (above ADC) is 1-2 OPEN, 2-3 SHORTED
Set SPAN (RV2) so TP3-TP4 is 1.0V
Set ADC midpoint (RV1) so TP5-TP4 is 0.0V
Remove D5, D6 to use ADC5540
R11 ommited
C54 shorted
Take care about C39 PCB errata
Changed R3 to 1K
Replaced C27 with 100pF/1K series RC cirtuit
I have double soldered all parts
I think i have done everything anyone should have done but i am having a problem

When the input voltage is between 0 and 2.5 Volts DC i get a strange noise. Its pixel height is indepented of the voltage scale i select.

I have the same problem when i apply a periodic waveform which has an applitude of about 0 and 5 Volts. Then, the waveform is OK only above 2.5 Volts

The strange in this story is that if i pause the application (Scopal, Labview e.t.c) and restart it, occasionally the other channel is having the same problem (One channel per time is having problem)

What the f...ck can be wrong? Has anyone been experienced with something like this.

Please help me

Thx in advance
 

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